With Sunwell Naxxramas and Eye of Eternity releases, next content patch 3.1 - Secrets of Ulduar is coming soon! Secrets of Ulduar release date will be announced in the trail (May 2, 2018).
Ulduar and Furious Gladiator are going to be longer than the current content. Maybe around 6 months, so Call of the Crusade season would be launched in 2018 November/December.